Chem 334 - Summer 1998 - Organic Chemistry I
Dr. Carl C. Wamser Dr. Carl C. Wamseracid-base review
definitions of Ka , pKa
periodic trends in acid/base strength
state of ionization as a function of pH
nomenclature (haloalkanes)
physical properties and trends with increasing size of the halogen ( F , Cl , Br , I ) :
larger p orbitals used in bonding
longer bonds
weaker bonds
greater polarizability
greater London interactions
higher boiling points
polarity is about the same for all C-X bonds
NUCLEOPHILIC SUBSTITUTION
the general reaction: R-X + Nu- ---> R-Nu + X-
recognize different examples of nucleophiles and leaving groups
kinetics - second-order (the rate-determining step is bimolecular)
stereochemistry - backside attack (inversion at a chiral center)
stereospecific - one stereoisomer gives different product from another stereoisomer
stereoselective - one stereoisomer, of several possible, is preferentially formed as product
leaving group ability: I- > Br- > Cl- > F-
other good leaving groups (weak bases)
water as a leaving group (from protonated alcohols)
nucleophilicity: I- > Br- > Cl- > F-
solvation effects
other good nucleophiles (strong bases)
solvent effects: H-bonding diminishes nucleophilicity
alkyl group reactivity: CH3 > 1° > 2° > 3°
steric hindrance lowers reactivity
substitution at cycloalkyl halides is more difficult with small rings